1. Field of the Invention
The present invention relates to a split type connecting rod, in which a large end portion that has been subjected to surface hardening treatment is fractured and split into a rod portion and a cap portion, and the rod portion and the cap portion are recoupled using a coupling bolt while keeping the fractured surfaces of the respective portions aligned.
2. Description of the Related Art
When fracturing and splitting a large end portion into a rod portion and a cap portion, this kind of split type connecting rod may be provided with a fracture starting point groove for initiating the fracture. For example, as shown in FIG. 1A and FIG. 1B, a structure with a pair of fracture starting point grooves 51 which extend in the direction of the shaft center of a crank-pin hole 50a of a large end portion 50 notched and formed on the inner circumferential surface of the crank-pin hole 50a has been proposed (e.g., see U.S. Pat. No. 4,569,109).
On the other hand, when the large end portion 50 is fractured and split, the fractured surfaces advance as shown in FIG. 2A and FIG. 2B from the fracture starting point groove 51 along paths indicated by arrows X and Y around the bolt holes and the paths will meet each other at an exterior wall portion Z.
However, if the fractured surfaces deviate while advancing along the paths indicated by arrows X and Y, the fractured surfaces fail to meet on the same plane crossing each other at the exterior wall portion Z, causing a level difference and cracking. This may cause peeling or falling of the fracture surfaces, which may damage the engine parts while the engine is running, causing engine trouble.
Increasing the stress intensity factor of a bottom portion of the fracture starting point groove is effective for preventing the problem of peeling or falling. Thus, it is possible, as shown in FIG. 3, to increase the groove depth L2 of the fracture starting point groove 51 and increase the ratio of the groove depth L2 to the shortest distance L1 between the fracture starting point groove 51 and the edge of the bolt hole 50b. 
However, increasing the groove depth L2 of the fracture starting point groove 51 causes many problems described in the following paragraph.
First, when surface hardening treatment such as carburization is applied to the connecting rod, the carburization may not reach the bottom portion of the fracture starting point groove and the surface hardened layer at the bottom portion may be shallow. As a result, the fracture starting point cannot be hardened enough, and therefore the surface may be easily transformed into a surface with ductile fractures and the accuracy of alignment between the rod portion and cap portion after the fracture may deteriorate.
Second, as the groove becomes deeper, the processing time required for machining of the groove increases and it is more difficult to control the groove depth accurately. This problem becomes particularly noticeable when groove machining is performed using a laser.